The mu opioid receptor is a G protein coupled receptor expressed in the central and peripheral nervous system, and is activated by opioid compounds such as morphine. Such opioids have been used for centuries to provide effective pain relief and continue to be essential tools in modem clinical pain management. However, the clinical utility of opioid drugs is limited by side effects including gastrointestinal complications, respiratory depression. The clinical utility of opioid drugs is further'limited by the possibility that patients will develop tolerance or dependency over long-term use. Endogenous opioids, such as the carboxy-amidated tetrapeptide, endomorphin-2 (EM-2), binds the mu opioid receptor with high affinity and are analgesic in several animal models of pain. However, while endomorphin peptides have been isolated from bovine and human brain, no gene sequences corresponding to a potential preproendomorphin gene have been identified in human genome sequence databases, which renders the production and use of endomorphins problematic. Accordingly, there is a need for a genetic expression system for expressing carboxy-aminated peptides, such as endomorphins.